Acoustic resonators (also called “acoustic filters”) can be used for filtering high-frequency signal waves. Using a piezoelectric material as a vibrating medium, acoustic resonators operate by transforming an electrical signal wave that is propagating along an electrical conductor into an acoustic signal wave that is propagating via the piezoelectric material. The acoustic signal wave propagates at a velocity having a magnitude that is significantly less than that of the propagation velocity of the electrical signal wave. Generally, the magnitude of the propagation velocity of a signal wave is proportional to a size of a wavelength of the signal wave. Consequently, after conversion of an electrical signal into an acoustic signal, the wavelength of the acoustic signal wave is significantly smaller. The resulting smaller wavelength of the acoustic signal enables filtering to be performed using a smaller filter device. This permits acoustic resonators to be used in electronic devices having size constraints, such as cellular phones and smart watches.
Bulk acoustic wave (also called “BAW” or “volume”) resonators are part of a type of acoustic resonators manufactured in a sandwich construction. The sandwich construction includes a piezoelectric material positioned between an overlap of two electrodes in an active region of the BAW resonator. One of the electrodes is coupled to an electrode feed to provide an input signal for filtering. The other of the two electrodes is coupled to another electrode feed for communicating a filtered portion of the input signal to another electrical component.
Unfortunately, an outer perimeter of the BAW resonator facilitates propagation of spurious lateral modes. This occurs because the piezoelectric material is free to oscillate in regions outside of the BAW resonator (e.g., outside of the overlap of the two electrodes). The propagation of spurious lateral modes causes the active region of the BAW resonator to deviate from an optimum vertical vibration (also called a “piston mode”), which causes a loss of energy and a reduction in resonator quality (e.g., a Q-score). Accordingly, designers strive to suppress lateral wave propagation by modifying boundary conditions of the BAW resonator. For example, a BAW resonator may be designed with a frame structure near the outer perimeter of the overlap of the two electrodes to suppress lateral modes. However, a frame structure may reduce electromagnetic coupling of the resonator and the resonator quality.
This background provides context for the disclosure. Unless otherwise indicated, material described in this section is not prior art to the claims in this disclosure and is not admitted to be prior art by inclusion in this section.